My (older white) 24" imac came with a default profile for the monitor called simply "iMac". I calibrated the display with my Spyder Pro colorimeter and noticed that Apple's default profile was not so bad and was a close but not exact match to the custom profile I made. If you don't have a hardware calibration device I'd stay with the default.
But if you care enough about color to be asking this you really should go out and buy a colorimeter. They've gotten cheap. The new Spyder Express is something like $80 now.
Before you spend either money on a hardware colorimeter or time using the calibration tools build into OS X give SuperCal (http://bergdesign.com/supercal/) a try.
I absolutely love the profiles I generated using it and it is definetly more obvious for my eyes to adjust the SuperCal samples than adjusting Apple's standard images.